The invention discloses a circuit breaker non full phase judging method based on a two-side voltage fundamental wave amplitude ratio. A protection device is used for measuring three-phase voltage and open delta voltage at the left side and the right side of the circuit breaker; fundamental wave amplitudes of the three-phase voltage and the open delta voltage at the left side and the right side of the circuit breaker and the ratio of the fundamental wave amplitudes of the open delta voltage at the left side and the right side are calculated respectively; when the fundamental wave amplitudes of the three-phase voltage at the left side and the right side are both larger than eta% of rated voltage, when the fundamental wave amplitudes of the open delta voltage at the left side and the right side are both larger than a fundamental wave amplitude threshold, and when the fundamental wave amplitude ratio of the open delta voltage at the left side and the right side is larger than a ratio fixed value threshold, circuit breaker non full phase is judged, and alarming or tripping is carried out after a short time delay t. The method does not rely on the magnitude of negative-sequence current and solves the problem that when non full phase happens to the circuit breaker in the case of light load and the previous negative sequence current criterion can not be recognized; and in addition, as the method does not need to judge a three-phase inconsistent contact of the circuit breaker, the non-full phase judging problem of a three-phase linked circuit breaker can also be solved.